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64
Concrete
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The properties of fresh concrete are important. They infl uence the handling, ease of
compaction and the uniformity of distribution of the concrete constituents, each of which
infl uences strength and durability of the hardened concrete.
Workability of concrete may be defi ned as that property of freshly mixed concrete which
determines the ease with which it can be mixed, placed, consolidated and fi nished to a
homogenous condition, ie workability must relate to the way the concrete is handled on-site.
The concrete must be capable of being transported by the designed method, must be easily
compacted, even into diffi cult sections, and must provide an acceptable surface fi nish, either
off-shutter or by power fl oating.
Workability is a composite property, and diffi cult to measure directly. It can, however, be
assessed in terms of consistence and cohesiveness:
• Consistence describes mobility or ease of fl ow and is related to the wetness of the mix.
Wetter concrete is usually more workable than dry concrete, but concretes of the same
consistency may vary in workability.
• Cohesiveness describes the tendency to resist segregation and bleeding.
There is a worldwide tendency to produce concrete of higher workability to facilitate
up the construction process. AfriSam Readymix produces “Flowcrete” with a slump
in excess of 200mm (fl ow of 500mm to 600mm), as well as self-compacting concrete
for special applications.
Concrete
Concrete is the
second-most widely
used material
on earth.
Whether readymixed or site batched, concrete is in a fresh state for only a few hours.
About three hours after water is added, concrete loses workability then gradually starts to set,
changing slowly from a plastic state into a rigid solid that with adequate curing will continue
to gain strength.
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Concrete, a sustainable resource
Sustainability is about balancing the associated economic, social and environmental factors, not only at
inception but during use.
From an economic viewpoint, although cement is costly to produce in both financial terms and in terms of embodied energy,
the amount of cement used in concrete is only about 10% of the total raw materials. And, in turn, the combined embodied
impact of cement, aggregates, water and admixtures used in concrete accounts for only 10% of the impact of the operating
phase of conventional buildings.
In the long-term, concrete’s durability, low maintenance and re-usability coupled with a myriad of other environmental
advantages have very positive long-term economic and environmental effects. In the construction industry this balance is of
great importance not only before and during the construction stage, but is also about making the right decisions at the design
stage, and choosing materials and construction methods to ensure long-term sustainability.
A model available freely from www.cnci.org takes into account “cradle-to-grave” emissions of common raw materials used
in concrete, including transport of those materials, and gives average emission numbers expressed in kg CO2 /ton of material
produced. By using this data, the designer can experiment with different material combinations to minimise the environmental
impact and quantify the effect of the material properties on cost per cubic metre of concrete.
For more about concrete’s innate cost-effectiveness, energy efficiency, thermal mass, light reflectance, fire resistance, low
maintenance, acoustic performance, pollution reduction, water conservation, construction flexibility, retrofitting, recycling
and re-use, see the C&CI’s series of booklets on Sustainable Concrete.
2Reducing our
Carbon Footprint
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Concrete
• Admixtures The use of water-reducing admixtures allows for
increased workability without increasing the water
content of the concrete. In some instances, a
considerable reduction in water content can be
achieved while maintaining workability.
See The use of admixtures in concrete.
Measuring consistence
The slump test is the universally accepted method of
measuring consistency. Other methods incorporated
into SANS standard test methods include the Vebe and
Compacting Factor tests for low-workability mixes and
the Flow test for high-workability concrete. With the
advent of self-compacting concrete, other tests such as
the slump fl ow, Y-funnel and L-box are frequently used.
Control of consistence
For a mix of given proportions and materials, consistency
is mainly affected by the water content, which in turn
affects the water:cement ratio and strength. Slump test
results have conformance limits, see Concrete specifi cation
requirements.
Factors affecting workability
Workability is affected by water content, actual
proportioning of raw materials, aggregate and cement
types and characteristics, admixtures, time elapsed after
mixing, and ambient and concrete temperatures.
• Water content In an average concrete mix using 19mm stone, a
total water content of about 210litres/m3 gives a
slump of 75mm. In a well-proportioned mix, an
increase in water content will make the concrete
more mobile or fl owable.
• Cement content and type Generally, mixes with low cement contents are less
workable and more diffi cult to fi nish; mixes with high
cement contents, typically above 500kg/m3, tend to
be sticky and lose workability quickly.
Using cements containing Fly Ash (FA) gives concrete
improved workability.
• Sand If the sand content is too low, the concrete will be
harsh. The sand content needs to be suffi ciently
high and contain about 30 to 40% material fi ner
than 300µm. Coarse sands are often blended
with fi ne sands to overcome this defi ciency.
• Aggregate characteristics The characteristics of stone and sand infl uencing
workability are shape, surface texture, average
particle size, grading and fi nes content.
• Roundedparticleswithasmoothsurfacetexture
improve workability (but in some instances may
be detrimental to strength).
• Theuseofstonewithasmallermaximumsize
improves workability, as does graded as opposed
to single-sized particles.
• Theuseofcontinuously-gradedasopposedto
single-sized sands improves workabiity.
Concrete has a “shelf life” of about
three hours from adding water to
initial set.
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The advantages of using readymixed concrete vs site-batching
• Thesupplierhastheresourcesandtechnicalexpertise
to provide a wide range of mixes.
• Thesuppliercanmoreeasilymeetchangestothe
construction programme.
• Betterqualitycontrol:computerisedweighbatching
offers better ultimate concrete performance.
• Time,labourandcost-effective:nopurchasing,
receiving, stockpiling of cement, stone and sand on
site: less pilferage.
• Concretecanbesuppliedtoseverallocationsatthe
same time.
• Lesslabourrequiredforloadingmixersandtransporting
concrete on-site.
• Speedofdischargemeetstightconstructiondeadlines
and high rates of delivery are possible.
• Thesuppliertakesresponsibilityforrawmaterialand
process control testing during the production process.
• Availabilityofbackupsupply.
• Reductionofrisk.
Although site batching is frequently seen as a more
economical option than readymixed concrete, the following
factors should be costed into site batching:
• Wastageandtheftofmaterials.
• Handlingandstorageofmaterials.
• Planthire/depreciation.
• Plantestablishmentandremoval.
• Plantoperation.
• Labour.
• Supervision.
• Technicalrequirements.
• Sitetransportequipmentavailabletoothertradesatall
times rather than tied up transporting concrete on-site.
Site-batched concrete is not subject to SANS 878 requirements.
Bleeding
Bleeding is a form of segregation in which some water
migrates to the surface as the solid particles settle. This
may result in a layer of clear, greenish water forming on the
surface of the concrete. This will continue until the concrete
has stiffened sufficiently to prevent further settlement.
The use of high extender contents and retarding admixtures
will prolong the setting time and thus increase the time
during which bleeding may occur.
Defects attributed to bleeding include:
• Formationofvoidsunderaggregateparticlesand
reinforcing steel.
• Sandstreakingresultingfromthebleedwaterrising
up the surface of formwork, taking fine particles of
sand and cement with it.
• Thetrowelling-inofbleedwateronthesurfaceofa
slab resulting in a weak dusty layer.
• Settlementcracking.
Bleeding may be reduced by:
• Increasingthebindercontent.
• UsingCondensedSilicaFume(CSF).
• Reducingthewatercontent.
• Increasingtheamountofminus300µmmaterialin
the sand.
• Usinganair-entrainingadmixture.
Basic production requirements are the same for both
site-batched and readymixed concrete. However, on a
daily basis the readymix producer accommodates:
• Agreatervarietyofplantandprocesstechnology.
• Awiderrangeandcombinationofcementsand/or
extenders, aggregates and admixtures.
• Varyingmixrequirementsinthefreshand
hardened states.
• Awidevarietyofspeciications.
• Predetermineddeliverytimes.
• Environmentalrestraints.
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Concrete
• Batching tolerances should be specifi ed, typically
as follows:
•Cement: Cumulatively by mass to within 2%.
• Aggregate: To within 3%.
• Admixtures: To within 2% or 50ml.
• Water: To within 2%.
• Plant and equipment Storage of raw materials: Design to minimise
segregation, contamination or deterioration.
Weighing equipment: Calibrate and check regularly,
with monitoring devices clearly visible to the operator.
Mixers (stationary or truck-mounted): Keep in good
repair, check ability to fully mix concrete within the
required time.
• Production and delivery Batch solid materials by mass. Batch liquids by mass
or volume. Make appropriate adjustments for moisture
in aggregates, particularly sand. Control amount of mix
water by measurement and maintenance of slump
within specfi ed tolerances. Concrete should be delivered
with suffi cient workability for placement and
compaction. Slump tolerances should be within the
specifi ed tolerance range for a period of 3 minutes from
arrival on site.
Quality control
The quality of readymixed concrete is controlled by
compliance with SANS 878, both during production
(process control) and acceptance (or compliance) control.
The principal elements of control include:
• Identifyingthepropertiesofsuitablerawmaterials
and monitoring these properties.
• Proportioningthesematerialstoproduceconcreteof
the required quality in the fresh and hardened states.
• Identifyingprocessvariabilitytoallowcorrecttarget
strengths to be achieved.
• Adequatesamplingandtesting.
• Statisticalevaluationofresults.
• Correctiveactionintheeventofnon-compliance.
In order to comply with SANS 878 requirements, the
following factors should be in place:
• Contract: Types of concrete mixes, whether designed,
prescribed, or designed with special requirements (such
as minimum cement content or maximum water:cement
ratio) together with the minimum required information,
should be supplied by both purchaser and supplier to
ensure that quotations accurately refl ect requirements.
• Materials should comply with the following specifi cations:
• Cements: SANS 50197-1.
• GGBFS: SANS 55197-1.
• FA: SANS 50450-1.
• CSF: SANS 53263-1.
• Aggregates: SANS 1083, or have a proven record
of satisfactory use in concrete.
• Chemical admixtures: International standards such
as ASTM C494/C494 -08a.
• Water: SANS 51008. Test if the quality is in doubt.
Where wash-out water is used in concrete, water
density should be closely monitored to restrict
solids content.
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It is common practice to use at least two cementitious
materials, two fi ne and coarse aggregate products and
more than one admixture in the production of concrete.
To control the combined effects imparted to concrete
by all these constituents, comprehensive quality control
programmes have become essential.
Specii ed slump, mm Tolerance, mm
50 and less -15 to +25
More than 50, up to 100 ± 25
More than 100 ± 40
Where applicable, air content tolerance: ±1.5%
Table 32: Slump tolerances.
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• Sampling and testing freshly mixed concrete:
Should be strictly in accordance with the relevant
standard test methods.
• Compressive strengths for process and
acceptance control
Process control tolerances: No individual result
should fall below the characteristic strength minus
3MPa, and the average of 30 valid cube results
should exceed the specified strength by at least
1.64 times the current standard deviation.
Acceptance control is carried out by the customer
on-site to verify process control.
Tolerances: No individual compressive strength result
should fall below the characteristic strength minus
3MPa, and the average of three consecutive and
overlapping results should be at least equal to the
specfied strength plus 2MPa.
If acceptance control values are not met, cores may be
taken to verify strengths. Cores are generally drilled to
verify the strength of defective concrete (ie potential
low strengths, honeycombing, cracks, etc). They may
also be drilled to verify the strength of concrete where
no other data is available, eg:
• Whendoingaconditionsurveytoevaluatethe
health of an existing or damaged structure.
• Whencubespecimensorresultsgomissing.
• Ifanownerwishestoimposeadditionalload
on an existing structure and needs to get some
idea of the inherent strength of the existing
concrete elements.
AfriSam Readymix process
control testing
All AfriSam Readymix plants have sampling protocols to
ensure that sufficient slump and compressive strength test
results are available for statistical analysis by an advanced
customised computer program, allowing full evaluation
of all results and indicating the status of concrete
performance on an ongoing basis. These results are
available on request.
To ensure that the delivered concrete meets the specified
requirements in the hardened state, the customer or
sub-contractor must take full responsibility for all
subsequent on-site actions (see Handling concrete on-site).
Figure 12: Accepting concrete on-site.
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Concrete
Sampling and testing guidelines
Sampling concrete (SANS 5861-2)
• Avoidtakingsamplesfromtheirstorlast10%ofthecontentsofthemixer.
• Donotallowthemixtodropthroughaheightofmorethan500mmbeforethesampleistaken.
• Ensurethatthesampleisatleast1.5timestheamountrequiredfortestspecimens.
Dimensions and tolerances of cubes (SANS 5860)
• Thetoleranceonthebasicdimensionbetweeneachfaceofaspecimenis1.0%.
• Thebasicdimensiond should be at least four times the maximum aggregate size.
• Load-bearingsurfacestobelattowithin0.0005d.
Making and curing test specimens (SANS 5861-3)
• Preparethreespecimenspertestperage.
• Ensurethoroughcompaction.
• Labeleachspecimenwithauniqueidentiication.
• Preventthetopsurfacefromdryingoutfortheirst24hours.
• Demouldthecubeandplaceunderwateratatemperatureof22to25ºC.
• Duringtransportingtothelaboratory,preventlossofmoistureanddamage.
Coring concrete on-site
Problems encountered during drilling may include:
• Shatteringalreadydistressedconcrete,withsubsequentlyloosenedconcreteparticlesjammingthebarrelofthecoredrill.
• Cuttingintorebar,whichwill“snatch”orbreak-offthecoredrill,resultinginskewed,banana-shapedortoo-shortcores.
• Drillingthroughprestressedand(particularlydangerous)post-tensionedconcrete,whichmaycausetheslabtocollapse,
or may result in a safety hazard due to the “catapulting” effect of the suddenly-released energy.
The inherent difi culties of drilling into defective concrete combined with often restricted access and other very
difi cult conditions on-site make the following pre-drilling checks very important:
• Gettheengineer’spermissiontocore-drillingmaycausestructuraldamage.
• Usethebestequipmentpossibleforcoring.
• Checkplansanduseametaldetector/electricalconductordetectortoensurethatyoudonotdrillintoelectricalconduits,
water pipes or areas where groundwater fl ooding may be a hazard.
• Checkforrebarusingacovermeter:theproblemoftenisnotidentifyingwheretherebaris,butratheridentifyingareas
where there is NO steel. If possible, try to drill into the area with the deepest covermeter reading.
• Theminimumcorediameterisideallyatleastfourtimesthesizeoftheaggregateintheconcrete,iea100mmcoreis
good for 19mm stone, but not where 37,5mm stone has been used.
• Unlesscoringfordurabilityindex(DI)testing,drillcorespecimensaslongaspossible:thesurfaceconcretemaynothave
been adequately cured. If necessary, cut away any reinforcing steel to reveal a representative specimen of pure concrete
from deep within the structure.
Use an experienced coring contractor; and during the drilling operation ensure that:
• Thedrillisanchoredbyatleastonerawlbolttoensurerigidity.
• Plentyofwaterisusedtolushoutthecorebarrel.
• Drillingproceedscautiously,carefully,diligentlyandwithgreatpatience.
• Coresarelabelledassoonastheyhavebeendrilled,andcoreidentiicationiscross-referencedtothebuildingplan.
• Fragmentsofshatteredcorespecimensarereassembledintheordertheywereextracted(ieastheyexistedwithinthe
structure) and stored in suitable, labelled core boxes.
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The production of concrete, whether site batched or
readymixed for delivery to a construction site, involves the
activities outlined in Figure 13.
Prior to initiating the production process on-site or
ordering readymixed concrete, it is necessary to check:
• Speciications,drawingsandbillofquantitiesforthe
performance requirements of the concrete.
• Methodsofhandlingfreshconcreteon-site,together
with other construction requirements, eg early strength
for post-tensioning.
Once the requirements have been identified, the contractor
or readymix supplier selects suitable raw materials,
calculates mix proportions and carries out trial mixes.
Materials and mix proportions usually require approval from
the site engineer.
Production activities
• Raw material speciication requirements
Cements: See Cement.
Stone and sand: See Aggregates.
• Materials handling and storage
An appropriate sampling and testing procedure for raw
materials should be in operation.
Cementitious products, aggregate and admixtures must
be checked as far as this is practical to ensure
compliance with the purchase order, both in terms of
quality and amount, before discharged into the correct
bay, bin, silo or store.
Storage and handling should minimise contamination,
segregation or deterioration.
Clean drinkable water should be available for use as
mixing water in concrete. If not, check suitabliity against
SANS 51008.
Concrete production
Figure 13: Producing concrete.
*To verify that the required
workability and strength
requirements will be met, ongoing
process control testing is carried out.
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Concrete
Batching by mass
Batching by mass is preferable to volume batching,
although liquids are often batched by volume.
Cementitious materials should be batched cumulatively in
the same hopper.
For aggregate, the amount of moisture, particularly in the
sand, must be taken into account when calculating the
amount of water required to attain the specifi ed slump
and W/C ratio.
Various mass measuring systems are available. Irrespective
of the system used, it is essential that all batching
equipment is routinely maintained, and that load cells or
scales are regularly calibrated and frequently checked to
ensure compliance with required batch tolerances (see
Quality control).
Batch instructions giving the correct amounts of each raw
material specifi ed by the mix design for individual mixes
must be available at the plant. Batch details are simple for
a site operation with few mixes, but more complex for a
readymix operation where a large number of mixes are
routinely available.
The batching operation may be manual, semi- or fully-
automatic. Manual batching is suitable for low production
rates, but for most applications semi- or fully-automatic
computerised systems are preferable. Interlocks should
be provided to ensure proper operation of the system
and traceability.
Effective stock and yield control is possible when using
batch computers capable of recording actual amounts of
material batched. Computerised management systems are
then used to analyse this data to generate automatically
downloaded batch exception warnings, correlate batch
weights with slump test and 28-day compressive strength
results, and allow for scientifi c mix optimisation.
Figure 14: Production equipment.
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Machine mixing
Mixing is usually carried out by a machine, the common
types being non-tilting, tilting, reversing drum, split drum,
horizontal shaft and pan mixer.
Materials are loaded in a specific sequence to minimise
mixing time, and a mixing time is established for the mixer
used. The mixing cycle includes time to charge, mix and
discharge the mixer.
Undermixing can increase the variability of the concrete
from a workability and strength perspective, but
overmixing has minimal effect.
Mixing is done until the concrete is of uniform consistence,
colour and texture. All batches should be inspected visually
prior to being released.
• Emptythemixercompletelyaftereachbatch.
• Cleanthemixer/drumthoroughlyafterdischarge.
Batching by volume
For mix proportions for low-, medium- and high-strength
concrete, request our DIY brochures on concreting,
bricklaying, brick and blockmaking, and plastering.
Generally, 19mm and 26,5mm stone sizes are commonly
available, but check with your supplier as stone sizes are
currently under review.
Only enough water should be added to give the required
consistency or slump. Adding extra (excessive) water will
reduce the concrete strength.
The overall strength of the concrete is significantly
influenced by the quality of the sand. Where possible,
single-sized sands and sands with excessive fine material
should not be used.
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Concrete
Adjusting mixes on-site
The mix proportions are calculated using average
materials.
Check the fi rst batch of concrete. If the mix is diffi cult to
compact and it is not possible to achieve a smooth fi nish,
the mix is probably too stony. If the mix is too sandy, the
wearing properties of fl at slabs may be reduced.
• Concreteistoostonyifstonesprotrudeabove
the surface when the concrete surface has been
fl oated. In this case, reduce the stone volume by
half a wheelbarrow and increase the sand by a
similar amount.
• Ifathicknessofmortarofmorethanafewmillimetres
is available at the surface when the concrete is fl oated,
the concrete is too sandy.
In this case, increase the stone content by half
a wheelbarrow and decrease the sand by a
similar amount.
Figure 15 gives examples using APC, 19mm stone and
crusher sand.
Figure 15: Mix adjustment.
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Good concrete practice on-site is essential to
ensure that:
• Thequalityofthefreshconcrete,whetherreadymixed
or site batched, is maintained.
• Thehardenedconcretereachesitsoptimalpotential
strength and durability.
On-site activities relating to handling concrete include
ordering the correct concrete (site batched or readymixed),
transporting, placing, compacting, finishing and curing.
The scale of these activities ranges from high-rise
buildings entailing the use of sophisticated equipment
such as concrete pumps, poker vibrators, power floating
and spray-on curing membranes to simple labour-intensive
low-cost housing, but the principles outlined here
are the same for all concreting activities on-site or at
precast yards.
An example of a site checklist is given below.
Ordering readymixed concrete
When an AfriSam customer requests and accepts a
quotation for Readymix concrete, a tentative date is booked
for delivery. When the customer confirms the date the site
will be ready to accept the concrete, loads are supplied
from AfriSam plants (generally from the plant/s closest to
site) at specified intervals, eg first load at 10:00 am, with
subsequent loads every hour thereafter.
To take full advantage of this service, the customer should
be aware of the following factors:
• Atruckmixertakesapproximately30minutesto
discharge a full load.
• Concreteforfoundationsisusuallypoureddirectlyfrom
the truck mixer chute into the trench.
• Makesurethatgoodaccessisprovidedandthatthe
edges of the trenches are firm enough to take the
weight of the fully-loaded truck, ie approximately
30 tons.
• Ifitisnecessarytomovetheconcreteon-siteby
wheelbarrow, 15 to 20 wheelbarrows will be required
for each cubic metre. Organise labour in advance.
• Ensure that site preparation is complete, eg required
formwork has been erected and is clean and adequately
supported to retain the mass of the concrete, and that
steel reinforcing is adequately secured before accepting
the concrete.
Handling concrete on-site
Table 33: Pre-concreting check list.
77
Concrete
Transporting concrete on-site
When selecting a suitable transporting
method, assess:
• Siteconditions.
• Availabilityofsiteequipmentsuchascranes,especially
where used for moving formwork, etc.
• Rateandvolumeofconcreting.
• Useofsite-batchedorreadymixedconcrete.
Whatever method is used to transport concrete, the
following points need consideration:
• Themethodmustbeappropriateforthetypeofmix.
• Hourlyratemustbecompatiblewithmixingand
placing operations.
• Transportingshouldbefastenoughtopreventdrying
out or loss of workability.
• Delaysmustbeminimisedtopreventtheformationof
cold joints.
• Thereshouldbenocontaminationofthemix.
• Segregation,includinglossofinematerial,mustbe
kept to a minimum.
Concrete used in suspended slabs to fi ll areas between
elements and as a topping is generally pumped. The precast
slab or block supplier will supply details regarding propping
and the depth of concrete specifi ed. Note that the heavy
pressures placed on support work by “wet” (fresh) concrete
make suffi cient, accurately vertical propping essential.
Placing
Ideally the time between mixing and placing a batch should
not exceed 45 minutes. During delays, if the workability
cannot be restored fully by turning the concrete over a few
times with spades, discard the batch.
Extra water added to the mix to restore workability
(retempering) weakens the concrete.
During placing, the aim is to maintain the quality and
uniformity of the concrete, ie prevent segregation.
• Forfoundations,dampentrenchesbeforeplacing
the concrete.
• Forreinforcedfoundations,ensurethatthereinforcing
is fi xed fi rmly to avoid displacement during pouring.
Use spacers to lift the steel off the bottom of the
trench. Consider pumping concrete into place to ensure
adequate cover to reinforcement.
• Fornon-wearingloorslabs,placetheconcrete
onto well-compacted and slightly damp fi ll (no
standing water).
• Fordriveways(andlargeslabs),dividetheareainto
panels, eg 3m by 3m (not more than 4.5m by 4.5m)
to prevent the formation of unsightly cracks due to
contraction during hardening. Lay alternate panels
(1, 3 and 5) on the fi rst day, then remove crossforms
and lay fi ll-in panels against the hardened concrete
the next day or later. Place the concrete as close as
possible to its fi nal position, and work the concrete
right into the corners and along edges with a spade
or trowel.
• Checkforsegregationwhenconcreteisdischarged
from a skip, chute or conveyor.
• Checkfordamageordisplacementofreinforcement,
stressing ducts and formwork.
Flash set and false set: see Cement.
If the concrete is to be placed a considerable distance from where the truck is parked, consider pumping. In this case, order the concrete and the pump at least two weeksin advance.
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The pump operation The pump rig arrives on-site about 30 minutes prior
to scheduled pour, and the pump operator sets up the
equipment. A truck-mounted pump and truck mixers
need good, firm access roads into the site. A truck
mixer loaded with 6m3 weighs 30t. The vehicle is 8m long
and 2.5m wide.
The pump, boom and pipeline are primed, and the slush
in discarded. Pumping commences within 15 minutes of
discharge of the readymix concrete into the pump hopper.
Pumping concrete
The advantages of pumping concrete include:
• Placingconcretefaster.Pumpedconcreteislowable,
yet highly cohesive, allowing for easy placing,
compaction and finishing.
• Placingconcreteinareasnotreadilyaccessible,eg
into heavily-reinforced elements, underground or for
high-rise buildings.
• Convenience.Onresidentialbuildingsites,norampsare
required to move concrete to first floor decks, and there
is no need to break down garden walls to allow truck
mixer access.
In addition to the normal procedure for ordering concrete,
the customer needs to take the following factors into
account:
• MaintainclosecommunicationwiththeReadymix
company/site batch operator throughout the
pumping process.
• Orderthepumpandthereadymixconcreteatleast
two weeks in advance, confirm date and time of pour
72 hours in advance.
• Liaisewithconcretesupplierifthesitewillnotbe
ready in time to start the scheduled pump job (to
within 30 minutes).
• Wheredifferentconcretestrengthsarerequiredfor
different elements, supply a marked-up site plan
indicating placing requirements.
• Forlargerormorecomplicatedpours,apre-site
inspection may be arranged to assess access, pump
and pipeline requirements, specific safety aspects on
site and special site requirements.
“Hidden” costs
• For tenders, include the cost of setting up the pump.
If a static pump is required, a foundation may be required.
• Toensureminimalblockages,thepumpandpipelineare
lubricated with a priming slush immediately prior to
pumping the first load.
• Additionalcompactingandinishingequipmentmaybe
required as concrete is discharged faster by pumping.
Figure 16: Boom reach.
79
Concrete
The pump operator is in charge of the entire pump
operation, including:
• Pump equipment. Only the pump operator is
authorised to operate the pump.
• Communication with the hose-handling crew with
regard to boom position, rate of discharge and
“breaking-back” pipe segments.
• Communication with the concrete supplier with regard
to delivery rate of concrete to site, and, where required,
return of unpumpable concrete to the plant.
• Locating and clearing blockages. Note that long
delays may result in emptying, washing out and
repriming the pump, boom and pipeline.
• Relaying last load (“i nals”) requirements to the
supplier. To avoid costly delays, estimate this while the
third-last truck is discharging.
• Presenting delivery notes collected from each truck
driver to the site representative for checking and
signing. Invoicing is for the amount of concrete
delivered at the rate quoted, plus set-up costs.
The pump operator and crew place the concrete as close
as possible to the fi nal position. If it is necessary to move
the concrete by shovel, labourers should not be allowed to
throw the concrete into place or use poker vibrators to prod
the concrete into place.
Clean-out area
Truck chutes, pumps and pipelines should be washed
out or cleaned with compressed air (where available)
in a designated area. No washout water should drain
into sewage systems – if necessary, prepare a
sandbagged area.
Compaction
After placing, compact and fi nish the surface of the
concrete prior to initial hardening. Entrapped air reduces
the concrete strength, eg 4% air voids may cause 25%
reduction in potential strength. Full compaction of
concrete maximises strength and impermeability, and
ensures a good off-shutter fi nish.
Before compaction, ensure that:
• Formsaretight-ittingtoavoidlossofgrout.
• Depthsofverticalsectionsareshallowenoughto
ensure complete compaction of each layer.
For successful hand-compaction by tamping or rodding,
concrete slump should be at least 100mm.
Mechanical vibration is usually carried out on larger jobs,
using internal (poker) vibrators, surface or vibrating beams
for fl oor slabs and sometimes form-mounted vibrators in
precast yards.
Stiff (low slump) mixes contain more air than high slump
concrete and therefore require more compactive effort.
Using poker vibrators, do not over-vibrate: insert the
vibrator at about 400mm intervals and compact for
10 to 15 seconds.
To ensure a dense and durable surface, slabs must be
well compacted, either using vibrating beams or a timber
beam with a tamping and sawing motion, with additional
compaction at edges and corners.
Finishing
Floors to be carpeted or tiled should be as smooth and level
as possible using wooden fl oats and avoiding over-working.
At no stage should neat cement or cement: sand mixtures
be trowelled into the surface to soak up bleed water.
Driveways: Use a wood fl oat or a hard brush to texture
the surface.
Steel trowelling gives hard, smooth fi nishes, eg for
industrial fl oors, parking garages, etc.
• High pressures are used to force the concrete through the pipeline. Only staff involved in the pumping operation are allowed in the area, and no one is allowed under the boom.
• If conditions are unsafe, the pump operator is authorised to terminate the pump operation.
80
Curing
To allow the concrete to reach its full potential strength,
adequate curing is essential. Curing maintains a satisfactory
moisture content and a favourable temperature in the
concrete to ensure ongoing hydration of the cement and
thus development of optimal strength and durability.
Commence curing activities immediately after completion of
surface finishing.
• Keepthetopsurfaceoftrenchesdampbycoveringwith
plastic sheeting for seven days or until building starts
(the bottom and both sides of the concrete are
essentially in a self-curing environment).
• Floorslabs,drivewaysandsuspendedslabsshouldbe
subjected to continuous curing (plastic sheeting, damp
sacking or damp clean sand, or continuous spraying)
for seven days for a durable, wear-resistant surface in
addition to maximum compressive strength.
This also applies to pumped concrete, even though the
concrete may appear to be wetter than normal.
To avoid the (temporary) variations in colour that tend
to occur when plastic sheeting is laid directly on a wet
concrete surface, the sheeting may be supported clear
of the surface by timber battens for the first 24 hours
of curing.
Wind must not be allowed to blow under the sheeting.
Light foot traffic may be allowed over new work 24 hours
after finishing, provided that the plastic sheeting is not
damaged or displaced.
Curing may also be accomplished by applying a fine mist
spray or curing compound, immersing the concrete
element in water or delaying removal of formwork.
Steel trowelling must not start until:
• Bleedingofthemixhasceased.
• Allbleedwateronthesurfacehasevaporatedor
been removed.
• Thesurfacehasstartedtostiffen.
Only then should steel trowels be applied, using
considerable pressure on the tools. Several trowellings
spread over a period of up to two hours may be required.
For large areas, power-operated machines should be
used. Trowelling should continue until the surface has
attained an even, fine matt finish. Only if a “polished”
finish is specifically required should trowelling be
continued thereafter.
Small amounts of water flicked on with a brush may be
applied to the surface to aid finishing but, as this tends to
weaken the surface, it should be done as little as possible
and only where trowelling alone does not produce the
desired results.
Planning of the work should take into account that the
delay period before steel trowelling can start is likely to
be two to three hours and longer in cold weather.
During the delay period, drying of the mix (as opposed
to evaporation of bleed water) must be avoided as this
may lead to cracking.
For hard non-slip areas, steel trowel as above and subsequently lightly texture the surface with carpet-faced loats or soft brushes.
• Use a barrier cream on hands when
handling fresh concrete.
• Wear protective footwear when laying loors.
81
Concrete
Special handling considerations
Self-compacting concrete (SSC)SSC requires a prolonged mixing time due to reduced frictional forces and to:
• Fullyactivatethesuperplasticiser.
• Improveddispersionofthehighammountofinesinthemix.
On arrival on-site, check workability retention.
Self-levelling concrete (eg Flowcrete)
• Increasedpumpingrates,increasedspeedofcastingandlowerviscosityofself-levellingconcreteplacegreaterlateral.
pressure on formwork. This should be addressed prior to formwork erection on-site.
• Inaddition,specialattentionmustbepaidtosealingformworkjointstoavoidgroutleakage.
• Forclosedelementsandnarrowsections,pointsforairexpulsionmustbeprovided.
Fibre-reinforced concrete (eg Polyi bre Mix)
• Special attention must be paid to adequate fi nishing of the surface and to joint detailing.
Poolmix
Poolmix is an extremely dry mix, and requires special handling techniques including the following:
• HavePoolmixdeliveredearlyinthemorningtoallowenoughtimefortheconcretetobeusedbeforesettingandto
ensure that the pool can be packed in one day.
• Havesuficientlabouron-sitetopacktheentireshellatonetime,therebyeliminatingjoints.
• Curetheconcreteadequately.
Poolmix is thrown onto the fl oor and walls using shovels, then compacted and smoothed using hand tools. Use all the
concrete before it starts to set. Do not retemper by adding water.
It is important to place the shell in one continuous operation. Construction or cold joints are undesirable because they are
diffi cult to seal and may weaken the structure. It is necessary to maintain the correct wall thickness and ensure the concrete
is well compacted.
Poolmix i nishes
• Conventional plaster: Mix 1½ wheelbarrows of sand to one bag of cement. Only good quality, fairly coarse plaster sand
should be used.
• Paint: Use high quality acrylic pool paint made by a reputable manufacturer and apply this in accordance with their
recommendations. Epoxy paints give reasonable service, but recoating is diffi cult.
• Marblite pool plaster: Mix Marblite incorporating Plastomar additive with clean water to a stiff, workable plaster.
After the walls have been plastered, continue with the fl oor. The entire pool surface should be plastered in one day.
Immediately after fi nishing, protect the concrete by covering with plastic or a shadeport. The concrete must be kept wet for at
least a week to allow it to gain its potential strength.
82
Many readymixed concrete and site-batch operations use
GGBFS as part of the binder material (cement) in their mix
designs. Bulk GGBFS is delivered to the plant by tanker,
pumped into silos and then automatically weighed and
batched at the same time as the cement.
On its own, GGBFS will not hydrate on contact with water
or harden at the same rate as portland cement; it requires
the presence of an alkaline activator such as portland
cement to initiate its inherent cementitious reactions.
The hydration is similar to that of portland cement and
produces similar hydration products (see Chemistry of
portland cement), but is more complex. In the hardened
state a GGBFS-portland cement paste is denser than a
CEM I-paste, increasing the density and thus
impermeability of the concrete.
The advantages of using GGBFS in concrete, either as
complementary material in the cement or as part of the mix
proportions in a site blend, include improved durability as a
result of:
• Reduced permeability: As a result of slower early-age
strength development, the pore structure within the
concrete tends to be more refined, decreasing
permeability and providing a greater protective pore
ratio (see Figure 17).
Figure 17: Mantel’s model of the hydration of GGBFS.
• Improved freeze/thaw characteristics: Reduced pore
size and pore refinement improve freeze/thaw durability.
• Resistance to chemical attack or attack by
aggresive agents: Because the rate of chloride ion
diffusion through concrete is dependent on pore
structure, GGBFS gives concrete improved resistance
to chloride and sulphate attack. In addition, resistance
to sulphate and soft water attack is improved due to
reduction of the calcium hydroxide content.
The use of GGBFS in concrete
• Reduced potential for alkali-aggregate reaction
(AAR): Studies carried out by CSIR show that 30
to 40% replacement of CEM I with GGBFS prevents
deleterious expansion due to AAR by tying alkali
salts produced by cement hydration into the
insoluble CSH gel.
• Lower heat of hydration, and control of heat
differentials in mass concrete: Thermal cracking
relates to the differences in temperature as a result
of hydration between the core and the surface of
the concrete. Using GGBFS/CEM I mixes with
between 50/50 and 70/30 proportions reduces
the risk of thermal cracking by slowing and
minimising heat generation.
• Reduced creep and shrinkage: Studies indicate that
in normal-strength GGBFS-extended concretes with
adequate curing, concrete creep and shrinkage are
reduced, and the concrete has the ability to curb
higher strains.
See also Soil-stabilisation and Properties of hardened
concrete.
The use of supplementary cementitious materials such as
GGBFS in concrete affects site practice:
• Water:cement (W/C) ratio: Most cements require only
28% of their own mass in water for full hydration.
Anything over this amount is usually only required to
improve workability of the mix. For a given mix design,
the higher the water content, the higher the cement
content; also the more heat generated, the shrinkage
and the number of voids.
GGBFS particles have a smooth, even surface texture
and these concretes require less compactive effort
than CEM I concretes, thus providing scope for lower
water contents to achieve workability requirements.
• Curing: Good curing ensures the internal durability
of concrete, and also prevents the moisture loss
from the surface which can cause plastic-shrinkage
and surface cracks. When using mixes containing
complementary materials such as GGBFS,
the importance of adequate curing cannot be
over-emphasised.
8 3
Concrete
Condensed Silica Fume (CSF) is used in concrete to
improve impermeability, as well as abrasion and chemical
resistance of high-strength and high-durability concrete.
The main advantages of using CSF include:
• Improvedresistanceofsteelreinforcementtocorrosion
via improved concrete electrical resistivity.
• Improvedbondbetweenpasteandaggregate,with
reinforcing steel and with steel or polypropylene fi bres.
• Reducedwearonconcretingequipment:pumps,
moulds, mixers, etc.
The use of CSF is highly recommended for the
following applications:
• Structures exposed to marine and chemical environments.
• Powerstationsandhydro-electricplants.
• High-risebuildings.
• IndustrialloorsReadymixedconcretetobepumped.
• Miningandtunnelling.
• Motorwaybridgesanddams,seeAlkaliaggregate
reaction (Properties of hardened concrete).
• Precastconcreteindustry.
Due to its pozzolanic nature, CSF can be used to enhance
the qualities of both fresh and hardened concrete. This
improvement is due to the formation of additional Calcium
Silicate Hydrate (CSH) binder, through the reaction of the
Silica Fume with the free lime (Ca (OH)2) present in the
cement. Silica fume is very rich in silicon dioxide (> 85%).
Hydration
When water is added to portland cement, hydration takes
place. CSH is formed, and calcium hydroxide or free lime
is released as a by-product of the chemical reaction (see
Chemistry of portland cement). When CSF is included in a
concrete mixture, the reactive silicon dioxide (SiO2)
component reacts with calcium hydroxide to form
additional CSH.
In comparing CSF-modifi ed concrete to concrete containing
FA, we see that the higher effi ciency of CSF results in the
pozzolanic action being evident much earlier. Furthermore
there is a greater degree of strength gain achieved when
CSF is used. Typically, the main contribution to strength
development in CSF concrete at normal 100 000 spheres
per cement grain curing temperatures will take place from
about 3 to 28 days. Sensitivity to curing temperatures is less
pronounced in CSF concrete than in FA concrete.
The presence of CSF in concrete accelerates the
hydration of the cement, improving the bond between
the aggregate and the cement matrix, and producing a
denser paste microstructure.
Addition rates
The normal addition rates of CSF are between 6 and 10%
by weight of the cement content of the mix. In certain
shotcrete and gunite applications, this percentage has been
increased to between 12 to 15%, to make the mix even
more cohesive and further reduce the rebound.
Where the addition rate exceeds 6%, a superplasticiser is
recommended so that the required slump can be achieved
at the required water:cement ratio. The dosage rate of
superplasticiser ranges between 1 and 2% of the cement
content, depending on the degree of workability required.
CSF can be used either to replace an equal weight of
cement or it can be added over and above the existing
cement content. In very highly aggressive environments,
it is recommended that CSF is added in addition to the
existing cement in order to substantially increase the
chemical resistance and durability of the concrete.
Even though addition is more expensive than cement
replacement, the improvement in the long-term
performance of the concrete structures far outweighs
the slightly higher initial expenditure on cement.
The use of CSF in concrete
84
Effect on fresh concrete properties
• Water demand: Due to the high surface area of CSF
particles, water demand may be affected. However,
no significant effect on water demand has been
identified where less than 5% by mass of cement
is used.
• Workability: CSF has a thixotropic effect. Concrete is
more cohesive and less prone to segregation, with
improved pumpability and advantages in underwater
pours. In order to compensate for apparent loss of
slump, increase initial slump by 20mm to 50mm.
Ask for advice in the use of admixtures with CSF, and
measure workability by using the Vebe test method.
• Bleeding: Greatly reduced, almost eliminated. The high
surface area of the CSF particles takes up some of the
water which may bleed upwards, and the formation of
silica gel effectively blocks capillary pores.
• Plastic shrinkage: Take extra care to cover surfaces in
high ambient temperatures, low humidity and areas
where high wind speeds may be expected, to minimise
formation of plastic-shrinkage cracks. Carry out
finishing and tooling activities as soon as possible after
placing and compaction.
• Curing: Start curing the concrete as soon as possible
after finishing, and maintain adequate curing for at
least three to seven days to ensure that all the
combined advantages of using CSF are achieved.
• Setting times: CSF does not noticeably affect setting
times. Where admixtures are required, dosage may
require adjustment: carry out trial mixes and request
expert advice, eg larger dosages of air-entrainer are
required in CSF concrete.
Effect of CSF on hardened
concrete properties
• Porosity: CSF in a concrete mix refines the pore
structure of the hardened concrete, with the number of
large pores being significantly reduced.
• Impermeability: The addition of CSF makes hardened
concrete significantly less permeable, and thus more
resistant to chloride attack, freeze-thaw damage and
chemical deterioration.
• Cement paste/aggregate transition zone: CSF gives
greatly improved durability and enhanced strength to
the hardened concrete due to improvements to the
aggregate/paste transition zone.
• Structural advantages: Using CSF in concrete with
compressive strengths in excess of 80MPa allows
for increased spacing between bridge and support
columns, with potential modification of column
dimensions and reinforcement requirements. See also
Durability (Properties of hardened concrete).
• WhenworkingdirectlywithCSF,useanapproved
dust respirator.
• CSFdustirritatestheeyes.Irrigatewithlargeamounts
of water.
• Skincontactisnothazardous.
85
Concrete
Plasticisers and superplasticisers
Superplasticisers (SPs) or High Range Water Reducers
(HR-WRs) are water-soluble organic polymers used in
concrete, generally at low dosages (< 1% by mass of
cement), in order to:
• Increasetheworkabilityatagivenmixproportionto
enhance placing characteristics (workability) of
fresh concrete.
• Reducethemixingwateratagivencementcontent
and workability, to reduce the and therefore to increase
concrete strength and W/C durability.
• Reduceboththewaterandcementcontentsatagiven
workability and strength, to reduce the creep, drying
shrinkage and thermal strains caused by heat of
hydration in mass concrete structures.
• Providecohesive,lowviscosityconcretewith
extended workability and high fl uidity, and minimise
mix segregation.
• Reducerequirementsformechanicalvibrationfor
placing and compacting, thus reducing noise levels
on-site.
• Providesmoothshutterinishoncolumnsdespite
highly congested reinforcement.
• Inprecastconcreteproducts,aidingfastplacement
and quick mould turnaround time, while giving a
high-quality fi nish with reduced blemishes.
SPs are generally used to achieve a combination of
some or all of the above concrete properties. The high
reduction of water content considerably improves
density, impermeability, mechanical performance and
durability characteristics (chemical and physical) of
self-compacting concretes.
SPs are available as aqueous solutions to facilitate dispersal
in the mix. Accurate, reliable and automatic dispensing at
the batch plant is essential, as is controlling and monitoring
the mix.
Effects of superplasticisers on
fresh concrete
The aim with using SPs for self-compacting concrete is to
produce robust, non-sensitive mix designs that can be easily
implemented.
Where used in conjunction with dry batch plants, there
is little room for error as the mix design has to be correct
fi rst time. Technical advice from suppliers is essential in
evaluating available raw materials, selection of the
appropriate SP, and optimisation of mix design to meet
concrete and budget requirements. The admixture supplier
should be capable of matching SPs with the specifi c
cement chemistry in terms of soluble alkalis and sulphates.
SlumpDepending on the dosage and type of polymer, SPs can
reduce the water content for a given workability by up
to 35%.
The slump retention may last for about two hours, after
which the concrete reverts to its original consistency,
plasticity or workability. The rate of the slump loss
depends on various factors including:
• Typeofadmixture/s.
• Theinitialslump.
• Ambientandconcretetemperatures.
• Typeandchemicalcompositionofcement.
• Typeandamountofmineraladditions.
• Effectofanyotherchemicaladmixturesused
in the concrete.
Setting timesSPs generally retard the initial and the fi nal setting times
of concrete but this retardation is not excessive. The
retardation effect depends on the type and dosage of SP,
the type of cement and the amount of mineral components
present in the concrete. Where high amounts of FA or
GGBFS are present, SPs may cause excessive retardation.
The use of admixtures in concrete
86
Segregation
Segregation may be defined as differential concentration of
concrete raw materials resulting in non-uniform proportions
in the concrete mass, ie the mass is not homogenous.
With higher-workability concrete care must be taken to
proportion the materials correctly to minimise segregation.
In flowing concrete, segregation may occur if there is not
sufficient fine material present.
Air content
SPs generally increase the air content in concrete but the
amount of air entrained depends on the type and dosage.
Bleeding
Bleeding may be defined as the autogenous flow of
the mixing water and its emergence from newly placed
concrete caused by the settlement of solid materials within
the concrete mass. As SPs reduce the water content, there
is generally no undue bleeding observed in self-compacting
concrete. In most cases, bleeding is reduced.
Pumpability
SPs allow concrete to be pumped for long vertical or
horizontal distances. For horizontal applications, slump
flows from 600mm to 650mm are required for swift and
easy coverage of large surfaces and flat toppings. Vertical
applications require much “wetter” concrete, with 700mm
to 750mm slump flows.
Compatibility issues
To avoid adverse effects on concrete, SPs must be
compatible, ie perform well when used together with other
chemical admixtures and should be used with care. Not
all SPs perform well when they are pre-blended or used
together in the same concrete mix. SPs are sensitive to
the cement type and its aluminates, sulphates and alkali
contents. Trial mixes are always recommended prior to use
on-site.
Effects of other admixtures
• Acceleratorsspeedupthechemicalreactionofthe
cement and water and consequently also the rate of
setting or early strength gain in concrete.
• Retardersslowdownthechemicalreactionofthe
cement and water, leading to longer setting times
and slower initial strength gain.
• Air-entrainersintroducebubblesintothemixwhere
maximum protection against freezing and thawing
is required, and are also used to increase workability.
Figure 18: Fluidity effect of adding SPs.
Figure 19: Potential effect plasticisers on concrete.
87
Concrete
The rate of evaporation is affected by environmental factors
such as temperature, relative humidity and wind speed. The
cumulative effect of these factors can be assessed using the
nomograph shown in Figure 20.
A recent study indicates that daily temperature fl uctuations,
especially at early ages, contribute to thermal strain and the
formation of cracks as well as to the severity of cracking.
Plastic-shrinkage cracks are not always evident during
fi nishing operations and may only be discovered the next
day. They may form in a random manner or be roughly
parallel to each other (see Figure 21). The cracks are often
almost straight, and usually 300mm to 600mm long (but
can be from 25mm to 2m long) and up to 3mm wide at
the surface.
These cracks generally taper quickly over their depth but
may penetrate right through a concrete element, forming a
weakness which widens and/or extends due to subsequent
drying shrinkage and thermal movement, and may lead to
water penetration problems.
Cracks appearing in concrete within the fi rst few hours
after placing are early-age thermal shrinkage cracks,
plastic-shrinkage cracks or plastic settlement cracks. It is
necessary to identify the type of crack and possible factors
causing the cracks before applying measures to minimise
the problem.
Plastic-shrinkage cracks
Plastic-shrinkage cracks form while the concrete is still
plastic, ie has not set. They occur when the rate of
evaporation of moisture from the surface exceeds the
rate at which moisture is being supplied, ie via bleeding.
Concretes with low bleed potential (eg low-slump mixes
containing a high proportion of fi ne material such as fi ne
aggregate or CSF) are more prone to plastic-shrinkage
cracks, but high bleed characteristics may promote plastic
settlement cracking, crazing, delays in fi nishing processes
and greater long-term shrinkage. Retarded concrete is also
more prone to plastic-shrinkage cracking due to increased
time in the plastic state.
Minimising cracking
Figure 20: Effect of concrete and air temperatures, relative humidity and wind speed on the evaporation of surface moisture from concrete.
88
Plastic-shrinkage cracks rarely occur near the edges of a slab
where the concrete is free to move.
The key to minimising plastic-shrinkage cracking is
controlling the rate of drying of the surface:
• Dampen subgrade and formwork before placing concrete.
• Inhotweather,lowerthetemperatureofthe
fresh concrete.
• Protectsurfacesfromdryingout,egerectwindbreaks.
• Commencecuringregimepromptlyafterinishingand
continue for the specified period.
See also Concreting in adverse temperatures, and Polyfibre
Mix (Readymix).
Plastic settlement cracks
Plastic settlement cracks show a distinct pattern, typically
mirroring the pattern of the restraining elements such as
reinforcement. The cracks occur when concrete is plastic,
frequently while bleed water is still rising and covers the
surface, and tend to roughly follow the restraining
element or changes in the concrete section.
After concrete is placed, the solids settle downwards
and the mix water bleeds up to the surface. If there is
no restraint this merely results in a slight lowering of the
water:cement ratio at the concrete surface. If the concrete
is locally restrained from settling while the adjacent
concrete continues to settle, there is the potential for a
crack to form over the restraining element (see Figure 22).
A void may also form under the restraining element,
affecting the local bond.
Plastic settlement cracks can be quite wide at the surface,
but taper in width until they reach reinforcing steel or
other restraining elements. They seldom extend beyond
the restraint. In exposed conditions this may increase risk
of corrosion of the reinforcement and pose a threat to
durability. Cracks may develop further due to subsequent
drying shrinkage, leading to possible full-depth cracking of
the concrete member.
To minimise the risk of plastic settlement cracking:
• Adjustmixproportionstocontrolbleeding,eglower
slump, better cohesiveness.
• Increasetheratioofcovertoreinforcingbardiameter,
ie increase depth of cover or decrease the size of bars.
• Dampenthesubgradebeforeplacingconcretetoavoid
excessive loss of water from the base of the element.
• Fixformworkaccuratelyandrigidlytoavoidmovement
during placing.
• Placeconcreteindeepsectionsirstallowtosettle,then
place and compact top layers, ensuring that the two
layers blend together.
• Compacttheconcreteadequately.
• Curetheconcretepromptlyandadequately.
Revibrating the affected concrete at the right time can
eliminate settlement cracks, especially in columns and
deep sections. Where there is an abrupt change in section,
concreting can be planned to allow for settlement to occur
in the deeper section prior to concreting the shallower one.
Figure 21: Typical plastic-shrinkage cracks.
Figure 22: Typical plastic settlement cracks.
89
Concrete
Steps during batching and mixing to minimise
problems of hot weather:
• Usehigherextendercontents.
• Usesuitableretardingadmixture.
• Aggregateshouldbekeptcool,egbyshading
stockpiles. Coarse aggregate may be sprayed with
water, but spraying fi ne aggregate is not practical and
can lead to problems with adjustment of water content.
• Cementtemperaturehasaminimaleffectduetothe
low amounts used, but white silos tend to minimise the
effects of high temperatures.
• Thetemperatureofthemixingwaterhasasubstantial
effect on reducing concrete temperature, so keep water
as cool as possible. In extreme conditions some or all of
the mixing water may be replaced by crushed ice.
• Batchingplantsshouldbeshadedasfaraspossible,and
preferably painted white.
• Eficientmaterialshandlingwilllimittemperaturerise
during production.
• Thoughexpensive,someconcreteshavebeen
successfully cooled by the injection of liquid nitrogen.
On-site:
• Limittransporttimeandtakeappropriatestepsto
eliminate delays in handling.
• Asfaraspossibleshieldtheareatobeconcretedfrom
high winds and direct sunlight.
• Scheduleconcretingforthecoolerpartsoftheday.
• Provideadequatecuringassoonaspossible.
Estimating concrete temperature
The temperature T of the fresh concrete can be estimated
from the expression:
T = 0.22 (TaWa + TcWc) + TwWw
0.22 (Wa + Wc) + Ww
where:
T = temperature of material, °C
W = mass of material, kg
a, c and w = aggregate, cement and water
This formula is applicable to estimating concrete
temperature in hot and cold conditions.
Adverse temperatures which may affect the setting and
strength gain of concrete exist in the following conditions:
• Theaverageambienttemperatureexceeds35°C.
• Theambienttemperatureis25°Caccompaniedby:
• Lowrelativehumidityandhighwindvelocity.
• Solarradiation.
• Highconcretetemperatures.
• Theaverageambienttemperatureisexpectedtobe
below 5°C.
The following guidelines assist in using appropriate
techniques to minimise the adverse effects of extreme
weather conditions.
Hot-weather concreting
Hot-weather concreting is not an unusual or specialised
process as it is a common occurrence throughout the
country for some months of the year.
When concreting in hot weather, there is typically an
increased rate of water evaporation and thus slump loss
from the fresh concrete, giving rise to potential problems
during handling and fi nishing processes.
In addition:
• Settingtimestendtodecrease.
• Theremaybeasmallincreaseinwaterrequirement
and early strengths tend to be higher.
• Theremaybeanincreasedincidenceofplastic
shrinkage cracks.
Although it is often stated that later strengths may be
reduced, a recent study showed no strength reduction
for concrete temperatures ranging from 23 to 33°C using
various binder types.
Concrete in adverse temperatures
90
Cold-weather concreting
Although not as much of a problem as experienced in many
parts of the world, cold-weather concreting does occur in
South Africa on occasion, and failure to adequately protect
the concrete can result in substantial strength reduction.
The effect of concrete freezing at early ages depends on
whether the concrete has set, and what strength has been
achieved when freezing occurs.
• Ifconcretewhichhasnotsetisallowedtofreeze,
there will be an increase in volume due to the
expansion of mix water. After thawing, the concrete
will set with a high voids content.
• Iffreezingoccursbeforetheconcretehasattaineda
strength of 3MPa to 5MPa, expansion will cause
disruption of the microstructure and a substantial
reduction in strength and durability.
From the above, we can see that in extremely cold
conditions fresh concrete should be maintained at a
suitable temperature until a strength of about 5MPa has
been achieved.
From a practical point of view, concrete should have
a temperature in excess of 7°C at time of placing. To
achieve this, steps can be taken to increase the concrete
temperature, eg by:
• Protectingaggregatewithasuitablecovering.
• Heatingaggregatebysteaminjection.
• Heatingmixingwater.
• Limitingtransportingtime.
On-site steps need to be taken to:
• Avoidtemperaturelossduetoslowplacing.
• Applyprotectionmeasurestomaintainthetemperature
of the placed concrete. These may include the use of
insulated formwork, covering exposed surfaces with
insulation material or the erection of covers with
internal heating.
• Delayinishingactivitiessuchaspowerloatingdueto
the longer stiffening times.
91
Concrete
Aggregate
The aggregate used for concrete fl oors infl uences:
• Potentialabrasionresistance.
• Dryingshrinkageoftheconcrete.
• “Saw-ability”ofjoints,iepreventionofravellingand/or
plucking during cutting. The harder the aggregate, the
higher the early strength requirement.
Table 34: Strength requirement for early joint cutting for different aggregate types.
Other important properties for aggregate used in
concrete industrial l oors are:
• 10% FACT values: The aggregate used for
concrete subjected to abrasion should comply
with SANS 1083 requirements.
• Bleeding of concrete: The amount of bleed water
is infl uenced by the grading and particularly the
fi neness of the sand used. If a concrete bleeds
excessively and the bleed water is trowelled in, the
surface W/C ratio is lowered and this may result in a
loss of abrasion resistance. When bleeding is likely to
be excessive, the use of a suitable fi ne blending sand
should be considered.
• Drying shrinkage: Drying shrinkage is infl uenced
by the type and source of aggregate. All AfriSam
aggregates have a history of suitability for use in
industrial fl oors. With regard to other properties,
aggregate used for concrete fl oors must comply
with the requirements of SANS 1083.
Concrete is used for industrial fl oors because of high
wear resistance, adequate fl exural strength and good
dimensional stability.
These properties are dependent on and infl uenced by:
• Theselectionandproportionsofconcretematerials.
• Thehandlingoftheconcreteinthefreshandearly
stages of hardening.
• Appropriateinishingandeffectivecuring.
Selection of concrete materials
Cement
In fl oors with sawn joints, concrete must achieve a certain
strength to allow for sawing of joints. The longer the period
between casting and saw-cutting, the greater the possible
moisture loss from the concrete and the higher the risk
of shrinkage cracks occurring before the concrete can be
sawn. This is also dependent on the effectiveness of curing.
To prevent such cracking, only cements with a relatively
high early strength should be used in concrete for fl oors.
High extender contents should be avoided as they
reduce the early strength of the concrete, and concrete
containing high extender contents requires signifi cantly
more effective curing for longer periods to ensure
adequate abrasion resistance.
The following AfriSam High Strength Cements are
recommended:
• CEM II A-M (L) 52,5N
• CEM II A-M (V-L) 42,5R
Industrial fl oors are often subjected to potentially aggressive
agents such as sulphates, acids, chlorides and abrasion.
Select a cementitious material that will improve the
resistance of the concrete to these aggressive agents.
See also Sulphate resistance.
Concrete industrial l oors
Aggregate type Early strength requiredbefore sawing
Granite or quartziteDolerite or andesiteFelsite
3MPa - 5MPa4MPa - 6MPa
> 8MPa
92
Admixtures
The use of chemical admixtures may improve the
properties of concrete. However, their use should be based
on an evaluation of their effects on specific materials
and combinations of materials, including strength
development, particularly within the first 24 hours
after casting.
In environments with high evaporation rates, concretes with
delayed strength development should be avoided.
See also The use of admixtures in concrete.
Handling concrete in the fresh state
and during hardening
All good intentions and efforts put into the mix
selection and proportioning may be wasted if placing
and compaction requirements are not adequately
addressed (see Handling concrete on-site).
It is almost always a combination of the following factors
that result in unexpectedly bad behaviour of concrete
in floors:
• Castingloorunderexposedconditions.
• Adverseambientconditions.
• Timingofinishingietooearly:powerloatingbefore
the concrete surface is hard enough, or too late: after
the concrete is no longer workable.
• Inappropriateinishingtechniques.
• Ineffectivecuring,alackofcuringorlateapplication
thereof, see also Curing.
• “Late”cuttingofcontractionjoints.
• Floorcastonplasticsheeting(notethattheuseof
plastic sheeting should be avoided).
• Theselectionofaninappropriateconcretemix
AfriSam Readymix recommends the use of
Surfacebed Mix for floors, see Readymix.
Other concrete mixes with lower early strengths have also
been used very successfully. However, if the ambient
conditions are adverse and/or curing is not started as early
as possible and/or is not effective, the concrete is more
likely to crack and the use of concrete with lower early
strength is a greater risk.
The importance of curing cannot be overstated. Most
problems investigated relate to ineffective curing. The
question to ask is not “Did the contractor cure the
concrete?” but rather “How did he cure the concrete?”
Detailed information about concrete industrial floors is
available from the C&CI.
What is effective curing? Preventing the loss of moisture from the concrete
from the time of placing for at least seven days
after casting.
93
Concrete
Heat of hydration
One of the main concerns with mass concrete pours is
the temperature rise (which may exceed 50°C) within
the concrete. See also Chemistry of portland cement.
In conjunction with the temperature rise, internal or
external thermal stresses are generated by restraint
to thermal movement.
• Internal restraint arises from temperature differentials
that occur when the concrete surface cools to ambient
conditions while the centre remains at a much higher
temperature. Cracks resulting from this temperature
differential may be external or internal.
• External cracks form when an excessive differential
occurs during the cooling phase.
• Internal cracks may develop if the differential is
exceeded during heating.
External restraint may be imposed by the immediate
environment such as a rigid base or adjacent pour.
This type of cracking is most common in walls cast
onto rigid foundations.
See also Thermal movement (Properties of
hardened concrete).
Mass concrete may be considered to be any volume of
concrete with dimensions large enough to require special
measures to minimise cracking by accommodating the heat
differential between core and surface temperatures, and
attendant volume change.
Generally, special precautions may need to be taken in
respect of heat of hydration for any pour with a least
dimension of 500mm.
Large pours, often in excess of 100m3, have become
common for structural applications such as raft foundations,
large bridge piers, nuclear pressure vessels, etc.
For large pours, attention to logistical and technical
considerations involves:
• Concretesupply.
• Castingsequence.
• Coldjoints.
• Heatofhydration.
• Early-agethermalcracking.
The principal benefi ts of mass pours are savings in cost
and time as a result of fewer joints and faster construction.
The disadvantages of cracks which might occur when
construction joints are not used appear to be minor.
Planning for mass pours
Planning considerations include:
• Concreteproductionandsupply.
• Usingconcretepumpstoallowforrapidplacingto
various parts of the pour.
• Labour.
• Placingsequence.
• Compatibilitybetweenrateofsupply,placement,
compaction and fi nishing.
Mass concrete on-site
Mass pours are ideal for readymixed concrete, as
concrete can be delivered from several batching plants
and scheduled to arrive on-site at a rate that ensures
continuous pouring.
94
Temperature rise
Factors which influence temperature rise include:
Cementitious material
The type and source of milled clinker component, the
use of mineral components (FA, GGBFS) and the total
cementitious content all affect the rate of heat generation
within mass concrete.
In general terms concrete should be designed to have
the lowest milled clinker content combined with the
highest mineral component content. However, as mineral
component content is increased, the total cementitious
content required to achieve the required compressive
strength may have to be increased.
Specifying compressive strength testing at later stages can
offset this to some degree.
Pour size, particularly minimum dimension
Maximum temperatures are generally recorded in the
centre of sections having a least dimension of about 2m.
To avoid excessive temperature differentials, the surface of
mass concrete elements is often covered with insulating
material, eg thermal blankets. Thinner concrete elements
lose heat more easily. A pour thickness of 1m will need to
be insulated for about five days, while a 4.5m thick section
will need insulating for 21 days.
Formwork type
Where plywood forms are used, even for relatively thin
sections, care must be taken to avoid thermal shock when
the formwork is removed, especially during winter.
Ambient and concrete temperature
Reducing the concrete placing temperature reduces the rate
of hydration and subsequently the peak temperature within
the mass concrete element. Specifications often limit peak
temperature to a maximum of 70°C.
Thermal strain
The thermal expansion coefficient of concrete is mainly
dependent on aggregate type. Siliceous aggregate has
higher coeffcients, dolerite and limestone lower values.
The thermal expansion coefficient of concrete is higher
than that of the aggregate itself.
Tensile strain capacity (crack resistance) also varies with
aggregate type. The expansion coefficient and tensile strain
capacity can modify the temperature differential, which will
cause cracking from 20°C for gravel aggregate to 39°C
for limestone.
Where possible, aggregate from a specific source should be
selected to give lower coefficients of thermal expansion of
the concrete.
See Aggregates and Thermal movement.
95
Concrete
Stabilisation is the process of mixing cementitious material
with granular material in pre-determined proportions to
improve the engineering properties of the granular material.
Compacting and curing the mix results in a bound material
with signifi cant strength results.
Adding a stabiliser to soil that is unsuitable for road
construction has economic benefi ts relating to elevating
sub-standard in-situ soil to comply with specifi c application
requirements. Strengthening the road subbase lower layers
can also result in cost savings in surfacing layers.
This section only refers to stabilisation with cement eg
Roadstab; stabilisation with lime and bitumen are beyond
the scope of this document.
Cement for soil stabilisation
Stabilisation projects are generally site-specifi c. Developing
a solution requires assessing the performance of the
in-situ material and using fundamental analysis and
design procedures to determine a cost-effective solution.
The selection of a cement type and content is then based
on laboratory testing with the granular materials and two
to three cement types available in the area of construction.
All laboratory testing should be carried out using standard
TMH1 and CSIR test methods.
Soil stabilisation
The availability of the cement type in the area
of construction should be coni rmed to prevent
unnecessary laboratory testing.
Please contact AfriSam for samples of suitable product
available in the area of construction, for pre-site trials.
Cement content
A minimum of 2% cementitious material is required to
ensure a uniform distribution of the stabilising agent
throughout the stabilised layer. Cement contents lower than
this may result in strengths not being achieved in practice
regardless of the results of laboratory testing.
The selection of the cement type infl uences the
“working time”, defi ned as the time between placing and
compaction of the stabilised layer (see Figure 23). Cement
starts hydrating as soon as it is in contact with moisture. If
most of the hydration has occurred by the time the material
is compacted, the chemical bonds that have been formed
between the cement and the soil will be broken down by
the compaction process and further chemical bonding will
be limited.
This limitation may result in lower in-situ strength of
stabilised layers.
Figure 23: Reduction of pH of in-situ material using different cement.
Cement A
Cement B
Cement C
16
14
12
10
8
6
4
2
0
0 1 2 3 4 5 6 7 8 9 10 11
PH
% of cement
96
Spreading
Distribution of cement can be done either by bag or bulk
spreading.
The uniformity of application of stabiliser needs to be
verifi ed by means of:
• Weighingtheamountofcementthatwasdeposited
onto a mat or tray placed at specifi ed intervals during
the spreading operation on the layer to be stabilised
(see Figure 24).
• Balancingthetotalamountofstabiliseragainstthe
specifi ed percentage of stabiliser and the stabilised area.
• Conirmingpercentageofstabiliserdepositedperarea
to be stabilised.
Figure 24: Amount of deposited stabiliser weighed to check coverage.
Compaction
Compaction should start immediately after fi nal mixing
and should be completed within the working time of the
stabiliser. The working time is infl uenced by the cement
type, soil type and ambient conditions. An indication of
working time may be obtained by establishing a strength
vs time relationship for the stabilised soil, as indicated in
Figure 25. The engineer may then decide on an acceptable
working time to limit the risk of strength loss.
Figure 25: Typical strength vs time relationship.
Curing
Curing is necessary to ensure that:
• Therequiredstrengthisachieved.
• Adequatewaterisavailableforhydration.
• Dryingshrinkageislimitedatearlystages.
The stabilised layer is cured for three to seven days after
construction to allow the layer to harden before subsequent
layers are placed.
Curing is done by means of:
• Maintainingthesurfaceinamoistconditionby
light sprinkling and rolling when necessary.
• Sealingthecompactedlayerwithabituminous
prime coat.
Cement with extended setting times, eg Roadstab
or a composite cement in the 32,5 strength class, is
suitable for soil stabilisation applications because
of the longer working times required to place and
compact the material.
97
Concrete
Applicable specii cations
ASTM C494 / C494M -11: Admixtures
BS EN 1992-1-1 2004: Structural use of concrete. Part 2: Code of practice for special circumstances (replaces BS 8110)
SANS 878:2012: Readymixed concrete
SANS 1083:2006: Aggregates from natural sources – Aggregates for concrete
SANS 1090:2009: Aggregates from natural sources – Fine aggregates for plaster and mortar
SANS 2001-CC1:2012: Construction works. Part CC1: Concrete works (structural)
SANS 1491-3:2006: Portland cement extenders. Part 3: Silica fume
SANS 10100-1:2000: The structural use of concrete. Part 1: Design
SANS 10100-2:1995: The structural use of concrete. Part 2: Materials and execution of work
SANS 50197:2000: Cement. Part 1: Composition, specifi cations and conformity criteria for common cements.
Part 2: Conformity evaluation
SANS 50450:2011: Fly Ash for concrete. Part 1: Defi nitions, specifi cations and conformity criteria.
Part 2: Conformity evaluation
SANS 51008:2006: Mixing water for concrete: Specifi cation for sampling, testing and assessing the suitability of water,
including water recovered from processes in the concrete industry, as mixing water for concrete
SANS 53263:2011: Silica Fume for concrete. Part 1: Defi nitions, specifi cations and conformity criteria.
Part 2: Conformity evaluation
SANS 55167:2011: Ground granulated blast furnace slag for use in concrete, mortar or grout. Part 1: Defi nitions,
specifi cations and conformity criteria. Part 2: Conformity evaluation
Test methods
SANS 5860:2006: Concrete tests – Dimensions, tolerances and uses of cast test specimens
SANS 5861:2006: Concrete tests. Part 1: Mixing fresh concrete in the laboratory.
Part 2: Sampling of freshly mixed concrete. Part 3: Making and curing of test specimens
SANS 5862:2006: Concrete tests – Consistence of freshly mixed concrete. Part 1: Slump test. Part 2: Flow test. Vebe test.
Part 4: Compacting factor and compaction index
SANS 5863:2006: Concrete tests – Compressive strength of hardened concrete
For road stabilisation:
Improved CSIR: Developed method: Determination of the Initial Consumption of cement required for road modifi cation
TMH1 Method A1(a): The wet preparation and sieve analysis of gravel, sand and soil samples
TMH1 Method A2, A3 and A4: Determination of the liquid limit, plastic limit, plasticity index and linear shrinkage of soils
TMH1 Method A7: Determination of the maximum dry density and optimum moisture content of gravel, soil and sand
TMH1 Method A8: Determination of the California Bearing Ratio of untreated soils and gravels
TMH1 Method A13T: Determination of the Unconfi ned Compressive Strength of soils and gravels
TMH1 Method A716T: Determination of the Indirect Tensile Strength of soils and gravels